1. Field of the Invention
Apparatuses consistent with the present invention relate to a small disc-loaded monopole antenna in which bandwidth is expanded by electromagnetic coupling of a shorted square disc that is coupled to a perpendicular ground plane with a folded strip line feed.
2. Description of the Related Art
With recent increases in the development of functions for mobile communications devices, movies or TV can be viewed through mobile communication terminals. Also, as additional functionality such as digital camera and playback of audio files, such as MP3 files, are added to the mobile communication terminals, the mobile communication terminals have become communication instruments as well as instruments which are utilized for leisure. Also, as multi-band, multi-functional communication terminals such as personal digital assistants (PDAs) that are capable of simultaneously using mobile communication functions and wireless local area network (WLAN) functions are developed, small, broadband antennas having gain characteristics are required which are capable of operating in several communication bands.
Monopole antennas and planar inverted F antennas (PIFAs) have been mainly studied as small internal antennas to be applied to currently used terminals. The PIFAs have maximum gains in directions that are perpendicular to planes of the PIFAs. Thus, in a case where mobile communication systems receive signals from unspecified directions, performances of the mobile communication systems such as sensitivities of telephonic communications, transmission speeds of data, or the like may vary greatly with orientation of the PIFAs. [For detailed contents of these antennas, refer to: Y. B. Kwon, J. I. Moon, and S. O. Park, “An internal triple-band inverted-F antenna,” IEEE Antennas Wireless Propagat. Lett., vol. 2, pp. 341-344, 2003; M. F. Abedin and M. Ali, “Modifying the ground plane and its effect on planar inverted-F antenna (PIFAs) for mobile phone handsets,” IEEE Antennas Wireless Propagat. Lett., vol. 2, pp. 226-229, 2003; and J. Fuhl, P. Nowak, and E. Bonek, “Improved internal antenna for hand-held terminals,” Electron. Lett., vol. 30, no. 22, pp. 1816-1818, October 1994.]
Therefore, monopole antennas having omni-directional radiation patterns are suitable for mobile communication terminals. However, since such a monopole antenna has a resonance length of 0.25λ, the monopole antenna must have a small structure to be used as an internal antenna. A method of transforming the monopole antenna into a folded type antenna is most widely used so as to make the monopole antenna small. [For detailed contents of these antennas, refer to: F. S. Chang, S. H. Yeh, and K. L. Wong, “Planar monopole in wrapped structure for low-profile GSM/DCS mobile phone antenna,” Electron. Lett., vol. 38, no. 11, pp. 499-500, May 2002; P. L. Teng and K. L. Wong, “Planar monopole folded into a compact structure for very-low-profile multi-band mobile phone antenna,” Microwave Opt. Technol. Lett., vol. 33, no. 1, pp. 22-25, April 2002; C. Y. Chiu, P. L. Teng, and K. L. Wong, “Shorted, folded planar monopole antenna for dual-band mobile phone,” Electron. Lett., vol. 39, no. 18, pp. 1301-1302, September 2003; B. Sun, Q. Liu, and H. Xie, “Compact monopole antenna for GSM/DCS operation of mobile handsets,” Electron. Lett., vol. 39, no. 22, pp. 1562-1563, October 2003; and K. L. Wong, Planar Antennas for Wireless Communications. New York: Wiley, 2003, pp. 26-71.]
Strip lines may be meandered, i.e., folded, so as to reduce physical sizes of antennas. However, bandwidths of the antennas are reduced. Thus, folded monopole antennas are mainly used as dual-band antennas by connecting monopoles that have different resonance lengths on feed lines. In another method of making monopole antennas compact, folded, shorted planar monopoles and feed patches are fed using an electromagnetic coupling force. [Refer to S. H. Yeh, Y. Y. Chen, and K. L. Wong, “A low-profile, bent and shorted planar monopole antenna with reduced backward radiation for mobile phones,” Microwave Opt. Technol. Lett., vol. 33, no. 2, pp. 146-147, April 2002.] A height of such a structure may be reduced to 0.1λ of a central frequency. However, a bandwidth is less than or equal to 10% of the central frequency.
Also, Δ-type multi-folded tapered strip lines have been recently suggested so as to reduce heights of antennas. [Refer to I. F. Chen and C. M. Chiang, “Multi-folded tapered monopole antenna for wideband mobile handset applications,” Electron. Lett., vol. 40, no. 10, pp. 577-578, May 2004.] In this structure, a height of an antenna is 0.09λ of a central frequency and a bandwidth of 13% of the central frequency. However, heights of the above-described monopole antennas can be reduced. Thus, the monopole antennas can be used as internal antennas. However, bandwidths of the monopole antennas are too narrow to be applied to wide-band communications.